Abstract:

According to one embodiment, an MRI apparatus includes a probe unit and a
control/imaging unit. The probe unit includes an probe, an converter, an
compressor and a transmitter. The control/imaging unit includes a
receiver, an expander and an reconstructor. The probe detects an RF echo
signal generated in a subject by a magnetic resonance phenomenon. The
converter digitizes the detected signal. The compressor compresses the
digitized signal in accordance with a predetermined compression parameter
to obtain a compressed signal. The transmitter generates a transmission
signal to wirelessly transmit the compressed echo signal and sends the
transmission signal to a radio channel. The receiver receives the
transmission signal and extracts the compressed signal from the received
signal. The expander expands the extracted compressed signal in
accordance with the parameter to obtain the RF echo signal. The
reconstructor generates a video signal regarding the subject on the basis
of the obtained signal.

Claims:

1. A magnetic resonance imaging apparatus comprising a probe unit and a
control/imaging unit, the probe unit including: an RF probe which detects
an RF echo signal generated in a subject by a magnetic resonance
phenomenon; an analog-digital converter which digitizes the RF echo
signal detected by the RF probe; an echo compressor which compresses the
RF echo signal digitized by the analog-digital converter in accordance
with a predetermined compression parameter to obtain a compressed echo
signal; and a first transmitter which generates a first transmission
signal to wirelessly transmit the compressed echo signal and sends the
first transmission signal to a first radio channel, the control/imaging
unit including: a first receiver which receives the first transmission
signal transmitted via the first radio channel and extracts the
compressed echo signal from the received first transmission signal; an
echo expander which expands the compressed echo signal extracted by the
first receiver in accordance with the compression parameter to obtain the
RF echo signal; and an image reconstructor which generates a video signal
regarding the subject on the basis of the RF echo signal obtained by the
echo expander.

2. The magnetic resonance imaging apparatus according to claim 1, further
comprising: a parameter determining unit which determines the compression
parameter, wherein the echo compressor and the echo expander perform
compression and expansion in accordance with the compression parameter
determined by the parameter determining unit, respectively.

3. The magnetic resonance imaging apparatus according to claim 2, wherein
the image reconstructor generates the video signal to indicate an image
showing the form of the subject, and calculates a signal-to-noise ratio
(SNR) of the image, and the parameter determining unit determines the
compression parameter in accordance with the SNR calculated by the image
reconstructor.

4. The magnetic resonance imaging apparatus according to claim 2, wherein
the parameter determining unit determines the compression parameter on
the basis of a part to be imaged and imaging conditions.

5. The magnetic resonance imaging apparatus according to claim 4, wherein
the image reconstructor generates the video signal to indicate an image
showing the form of the subject, and calculates a signal-to-noise ratio
(SNR) of the image, and the parameter determining unit determines the
compression parameter in accordance with the SNR calculated by the image
reconstructor.

6. The magnetic resonance imaging apparatus according to claim 2, wherein
the parameter determining unit is included in the control/imaging unit,
the control/imaging unit further includes a second transmitter which
generates a second transmission signal to wirelessly transmit the
compression parameter determined by the parameter determining unit and
sends the second transmission signal to a second radio channel, and the
probe unit further includes a second receiver which receives the second
transmission signal transmitted via the second radio channel and judges
the compression parameter from the received second transmission signal.

7. The magnetic resonance imaging apparatus according to claim 2, wherein
the parameter determining unit is determining at least one of parameters
regarding the RF echo signal as the compression parameter.

8. The magnetic resonance imaging apparatus according to claim 1, wherein
the control/imaging unit further includes a frequency down-conversion
unit which performs a frequency down-conversion of the RF echo signal
obtained by the echo expander, the echo compressor acquires the
compressed echo signal by decimating samples in accordance with a sample
decimating rate or sample decimating rule indicated by the compression
parameter out of the RF echo signal digitized by the analog-digital
converter, the echo expander inserts the sample thinned by the echo
compressor into the compressed echo signal extracted by the first
receiver in accordance with the sample decimating rate or sample
decimating rule indicated by the compression parameter, and then repeats
the estimation of amplitude and phase by use of frequency components in a
frequency domain defined by a center frequency and a bandwidth of an RF
pulse which is applied to the subject to cause the magnetic resonance
phenomenon, thereby acquiring the RF echo signal, and the image
reconstructor generates the video signal on the basis of the RF echo
signal which has undergone the frequency down-conversion in the frequency
down-conversion unit.

9. The magnetic resonance imaging apparatus according to claim 1, wherein
the echo compressor acquires the compressed echo signal by decimating
samples in accordance with sample decimating rate or sample decimating
rule indicated by the compression parameter out of the RF echo signal
digitized by the analog-digital converter, and the echo expander
estimates a pixel value of an image signal by reference to the sample
decimating rate or sample decimating rule indicated by the compression
parameter.

10. A method of imaging a subject by a magnetic resonance imaging
apparatus which comprises a probe unit and a control/imaging unit,
wherein in the probe unit, detecting an RF echo signal generated in the
subject by a magnetic resonance phenomenon, digitizing the detected RF
echo signal, compressing the digitized RF echo signal in accordance with
a predetermined compression parameter to obtain a compressed echo signal,
and generating a first transmission signal to wirelessly transmit the
compressed echo signal and sends the first transmission signal to a first
radio channel, and in the control/imaging unit: receiving the first
transmission signal transmitted via the first radio channel and extracts
the compressed echo signal from the received first transmission signal,
expanding the extracted compressed echo signal in accordance with the
compression parameter to obtain the RF echo signal, and generating a
video signal regarding the subject on the basis of the obtained RF echo
signal.

11. A probe unit used in a magnetic resonance imaging apparatus together
with a control/imaging unit, the probe unit comprising: an RF probe which
detects an RF echo signal generated in a subject by a magnetic resonance
phenomenon; an analog-digital converter which digitizes the RF echo
signal detected by the RF probe; an echo compressor which compresses the
RF echo signal digitized by the analog-digital converter in accordance
with a predetermined compression parameter to obtain a compressed echo
signal; and a first transmitter which generates a first transmission
signal to wirelessly transmit the compressed echo signal and sends the
first transmission signal to a first radio channel.

12. The magnetic resonance imaging apparatus according to claim 11,
further comprising: a parameter determining unit which determines at
least one of parameters regarding the RF echo signal as the compression
parameter, wherein the echo compressor performs compression in accordance
with the compression parameter determined by the parameter determining
unit.

13. A control/imaging unit used in an magnetic resonance imaging
apparatus together with a probe unit having functions of: digitizing an
RF echo signal generated in a subject by a magnetic resonance phenomenon,
compressing the digitized RF echo signal in accordance with a
predetermined compression parameter to obtain a compressed echo signal,
and generating a first transmission signal to wirelessly transmit the
compressed echo signal and sending the first transmission signal to a
first radio channel, the control/imaging unit comprising: a first
receiver which receives the first transmission signal transmitted via the
first radio channel and extracts the compressed echo signal from the
received first transmission signal; an echo expander which expands the
compressed echo signal extracted by the first receiver in accordance with
the compression parameter to obtain the RF echo signal; and an image
reconstructor which generates a video signal regarding the subject on the
basis of the RF echo signal obtained by the echo expander.

14. The magnetic resonance imaging apparatus according to claim 13,
further comprising: a parameter determining unit which determines at
least one of parameters regarding the RF echo signal as the compression
parameter, wherein the echo expander performs expansion in accordance
with the compression parameter determined by the parameter determining
unit.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priority
from Japanese Patent Application No. 2009-250917, filed Oct. 30, 2009;
the entire contents of which are incorporated herein by reference.

[0003] In a magnetic resonance imaging apparatus (MRI apparatus), a
detecting coil for receiving a magnetic resonance signal is located in an
imaging space within a gantry together with a subject. The magnetic
resonance signal detected by the detecting coil is generally transmitted
from the imaging space to an main unit (hereinafter referred to as a
control/imaging unit) via a cable extending to the outside of the gantry.
The control/imaging unit subjects the magnetic resonance signal to data
processing including image reconstruction processing and thereby images
information regarding the subject.

[0004] In such a general configuration, the cable is often an obstacle. As
disclosed in Jpn. Pat. Appln. KOKAI Publication No. 5-261083, it has been
conceived that, in order to avoid such a disadvantage, the magnetic
resonance signal is digitized by an analog-digital converter (ADC) in a
probe unit that includes the detecting coil called an RF probe, and then
converted into a radio signal and wirelessly transmitted to the
control/imaging unit by a data transmitter.

[0005] A sampling rate for digitizing an RF signal has to be twice the
frequency of an RF echo signal or more. Thus, if the RF echo signal is
digitized as it is, a high transmission data rate is required for the
data transmitter, and the power consumption of the whole probe unit is
increased accordingly. As the probe unit that is designed for wireless
use is activated by electric power supplied from a power source having a
limited power capacity such as a secondary battery, the power consumption
of the probe unit is desirably as low as possible.

[0006] It is therefore conceived to decrease the frequency of the RF echo
signal by frequency down-conversion to decrease the sampling rate so that
the transmission data rate required for the data transmitter may be
decreased accordingly. When the frequency down-conversion is used, the
frequency characteristic of a filter at the subsequent stage of a mixer
is designed to be fixed, and the frequency of a local signal to be
supplied to the mixer is desirably variable. However, it is not
preferable to install, in the probe unit, sophisticated hardware that
satisfies a high frequency resolution required for a frequency varying
function of the MRI.

[0007] Under such circumstances, it has been requested to dispense with
the frequency varying mechanism on the coil side, and at the same time to
alleviate the data rate requirement of the data transmitter to reduce the
load on the probe unit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIG. 1 is a diagram showing a magnetic resonance imaging apparatus
(MRI apparatus) according to first to fourth embodiments;

[0009] FIG. 2 is a block diagram of a detailed configuration of part of
the MRI apparatus according to the first embodiment;

[0010] FIG. 3 is a block diagram of a modified configuration of part of
the MRI apparatus according to the first embodiment;

[0011] FIG. 4 is a block diagram of a detailed configuration of part of
the MRI apparatus according to the second embodiment;

[0012] FIG. 5 is a block diagram of a modified configuration of part of
the MRI apparatus according to the second embodiment;

[0013] FIG. 6 is a block diagram of a detailed configuration of part of
the MRI apparatus according to the third embodiment;

[0014] FIG. 7 is a block diagram of a modified configuration of part of
the MRI apparatus according to the third embodiment;

[0015] FIG. 8 is a block diagram of a detailed configuration of part of
the MRI apparatus according to the fourth embodiment; and

[0016] FIG. 9 is a block diagram of a modified configuration of part of
the MRI apparatus according to the fourth embodiment.

DETAILED DESCRIPTION

[0017] In general, according to one embodiment, a magnetic resonance
imaging apparatus includes a probe unit and a control/imaging unit. The
probe unit includes an RF probe, an analog-digital converter, an echo
compressor and a first transmitter. The control/imaging unit includes a
first receiver, an echo expander and an image reconstructor. The RF probe
detects an RF echo signal generated in a subject by a magnetic resonance
phenomenon. The analog-digital converter digitizes the RF echo signal
detected by the RF probe. The echo compressor compresses the RF echo
signal digitized by the analog-digital converter in accordance with a
predetermined compression parameter to obtain a compressed echo signal.
The first transmitter generates a first transmission signal to wirelessly
transmit the compressed echo signal and sends the first transmission
signal to a first radio channel. The first receiver receives the first
transmission signal transmitted via the first radio channel and extracts
the compressed echo signal from the received first transmission signal.
The echo expander expands the compressed echo signal extracted by the
first receiver in accordance with the compression parameter to obtain the
RF echo signal. The image reconstructor generates a video signal
regarding the subject on the basis of the RF echo signal obtained by the
echo expander.

[0018] Hereinafter, embodiments will be described with reference to the
drawings.

[0019] FIG. 1 shows a magnetic resonance imaging apparatus (MRI apparatus)
100 according to the first to fourth embodiments.

[0021] The static magnet 11 has a hollow cylindrical shape, and generates
a uniform static magnetic field in its internal space. For example, a
permanent magnet or superconducting magnet is used as the static magnet
11. The gradient coil 12 has a hollow cylindrical shape, and is located
inside the static magnet 11. The gradient coil 12 has a combination of
three kinds of coils corresponding to X, Y, Z axes perpendicular to one
another. The gradient coil 12 generates a gradient magnetic field having
its intensity inclined along the X, Y, Z axes when the three kinds of
coils are separately supplied with currents from the gradient power
supply 18. Here, the Z axis is in the same direction as, for example, the
direction of the static magnetic field.

[0022] The gradient magnetic fields of the X, Y, Z axes correspond to, for
example, a slice selecting gradient magnetic field Gss, a phase encoding
gradient magnetic field Gpe and a read-out gradient magnetic field Gro,
respectively. The slice selecting gradient magnetic field Gss is used to
determine a given imaging section. The phase encoding gradient magnetic
field Gpe is used to change the phase of a magnetic resonance signal (RF
echo signal) in accordance with a spatial position. The read-out gradient
magnetic field Gro is used to change the frequency of the RF echo signal
in accordance with the spatial position.

[0023] A subject 200 is inserted into an internal space (referred to as an
imaging space) of the gradient coil 12 while being mounted on the top
board 17. The bed 16 moves the top board 17 in its longitudinal direction
(right-and-left direction in FIG. 1) and its vertical direction under the
control of the bed controller 21. Normally, the bed 16 is installed so
that the longitudinal direction of the top board 17 is parallel with the
central axis of the static magnet 11.

[0024] The RF coil unit 13 includes one or more coils contained in a
cylindrical case. The RF coil unit 13 is located inside the gradient coil
12. The RF coil unit 13 is supplied with a radio-frequency pulse (RF
pulse) adapted to Larmor frequency from the radio frequency transmitter
19 to generate a high-frequency magnetic field. The probe unit 14
includes at least one RF probe which is an RF coil for detecting a
magnetic resonance signal (RF echo) excited by the RF pulse. The probe
unit 14 is located on the top board 17. The probe unit 14 may otherwise
be incorporated in the top board 17. The probe unit 15 includes an RF
probe which is an RF coil for detecting the RF echo. The probe unit 15 is
attached to the subject 200.

[0025] The probe units 14, 15 are inserted into the imaging space together
with the subject 200 during imaging, and detect the RF echo generated by
a magnetic resonance phenomenon in the subject 200. Any types of probe
units are attachable as the probe units 14, 15. The probe unit 14 has a
function of sending the detected RF echo to the control/imaging unit 20
via a wired channel. The probe unit 15 is a unit independent of the main
body of the MRI apparatus 100. The probe unit 15 has a function of
sending the detected RF echo to the control/imaging unit 20 through a
radio channel.

[0026] The control/imaging unit 20 controls the gradient power supply 18
and the radio frequency transmitter 19 to generate a gradient magnetic
field and a high-frequency magnetic field in accordance with an imaging
sequence. The control/imaging unit 20 receives the RF echoes which have
been sent from the probe units 14, 15 and transmitted via the wired
channel and the radio channel, respectively. The control/imaging unit 20
then subjects the received RF echoes to data processing including image
reconstruction to generate a video signal of an image showing the form of
the inside of the subject 200 and the spectrum of the magnetic resonance
signal.

[0027] The display unit 22 displays the image on the basis of the video
signal generated in the control/imaging unit 20.

[0028] An instruction given by an operator is input to the operator input
unit 23. The operator input unit 23 provides the control/imaging unit 20
with a command that indicates the contents of the input instruction.

[0029] The general configuration of the MRI apparatus 100 is as described
above. Several embodiments that differ in the more detailed configuration
of the MRI apparatus 100 are described below.

First Embodiment

[0030] FIG. 2 is a block diagram of a detailed configuration of part of
the MRI apparatus 100 according to the first embodiment. In FIG. 2, a
characteristic configuration of the MRI apparatus 100 according to the
first embodiment is shown. Components which are not essential in the
first embodiment such as components associated with the control of the
gradient power supply 18 and the radio frequency transmitter 19 are not
shown.

[0031] The probe unit 15 according to the first embodiment includes an RF
probe 15a, an analog-digital converter (ADC) 15b, an echo compressor 15c,
a data transmitter 15d, a transmission antenna 15e, a reception antenna
15f, a parameter receiver 15g, a reception antenna 15h and a reference
signal receiver 15i. The data transmitter 15d and the transmission
antenna 15e configure a first transmitter. The reception antenna 15f and
the parameter receiver 15g configure a second transmitter. The reception
antenna 15h and the reference signal receiver 15i configure a third
transmitter.

[0032] The RF probe 15a receives an RF echo signal. The ADC 15b samples
and quantizes the RF echo signal in accordance with a sampling clock
input from the reference signal receiver 15i, thereby digitizing the RF
echo signal. In general, before input to the ADC 15b, the RF echo signal
received by the RF probe 15a is amplified by a preamplifier such as a low
noise amplifier (LNA) and filtered by a band pass filter (BPF). However,
the preamplifier and the BPF are not shown in FIG. 2. A sampling rate in
the ADC 15b is twice the maximum frequency of the RF echo signal or more.
The RF echo signal digitized by the ADC 15b is input to the echo
compressor 15c.

[0033] The echo compressor 15c compresses the RF echo signal input from
the ADC 15b by use of a compression parameter input from the parameter
receiver 15g. A known method for compressing a digital signal can be
properly used for the compression of the RF echo signal. For example, the
use of compressed sensing is assumed. In the case of the compressed
sensing, the compression parameter is a sample decimating rate or sample
decimating rule of the RF echo signals. In other words, a parameter
related to the RF echo signal is used as the compression parameter. When
the compression parameter is the sample decimating rate, the echo
compressor 15c generates a sample decimating rule for random extraction
from an input sample vector with a probability (1-sample decimating
rate). Accordingly, the echo compressor 15c inputs the sample vector
obtained by decimating samples out of the RF echo signal, to the data
transmitter 15d as a compressed echo signal. Alternatively, the echo
compressor 15c generates (number of input samples)×(1-sample
decimating rate) random number vectors having the same number of elements
as the input sample vector. The echo compressor 15c then inputs a sample
vector obtained by calculating an inner product of each random number
vector and the input sample, to the data transmitter 15d as a compressed
echo signal. When the compression parameter is the sample decimating
rule, the echo compressor 15c inputs a sample vector obtained by
decimating samples out of the input sample vector in accordance with the
sample decimating rule, to the data transmitter 15d as a compressed echo
signal.

[0034] The data transmitter 15d uses the compressed echo signal input from
the echo compressor 15c to perform error correcting
encoding/interleaving, modulation, frequency conversion, amplification
and filtering, thereby generating a first transmission signal. This first
transmission signal is supplied to the transmission antenna 15e and
thereby sent to the control/imaging unit 20 via a first radio channel
CH1.

[0035] A second transmission signal sent from the control/imaging unit 20
and transmitted via a second radio channel CH2 is received by the
reception antenna 15f, and input to the parameter receiver 15g. The
parameter receiver 15g subjects the second transmission signal to
amplification, frequency conversion, demodulation and
de-interleaving/error correcting decoding. As a result of such
processing, the parameter receiver 15g extracts a compression parameter
from the second transmission signal. The compression parameter is input
to the echo compressor 15c.

[0036] A third transmission signal sent from the control/imaging unit 20
and transmitted via a third radio channel CH3 is received by the
reception antenna 15h, and input to the reference signal receiver 15i.
The reference signal receiver 15i subjects the third transmission signal
to amplification, frequency conversion and demodulation. As a result of
such processing, the reference signal receiver 15i extracts a reference
clock from the third transmission signal. The reference clock is input to
the ADC 15b as a sampling clock. The reference clock may otherwise be
multiplied by a PLL before input to the ADC 15b.

[0038] The fixed frequency generator 20j is a device for generating a
reference clock signal which repeats amplitude changes at a given
frequency. The fixed frequency generator 20j is configured by, for
example, a quartz oscillator having a significantly high stability. The
reference clock signal is input to the variable frequency generator 20k
as an input clock signal. The reference clock signal is also input to the
reference signal transmitter 20m in order to synchronize the clock of the
probe unit 15 with that of the control/imaging unit 20. The reference
clock signal is also input to parts that require clock synchronization in
the control/imaging unit 20, such as the image reconstructor 20e and the
pulse waveform generator 20p.

[0039] The variable frequency generator 20k is a device which is activated
by the reference clock signal input from the fixed frequency generator
20j and which generates a clock signal (local signal) having a variable
frequency corresponding to a center frequency set value input from the
sequence controller 20f. The variable frequency generator 20k comprises a
phase-locked loop (PLL), a direct digital synthesizer (DDS) and a mixer.
The local signal generated by the variable frequency generator 20k and
having the variable frequency is input to the frequency down-conversion
unit 20d and the frequency up-conversion unit 20r.

[0040] In the control/imaging unit 20, the first transmission signal
transmitted from the probe unit 15 via the first radio channel CH1 is
received by the reception antenna 20a, and input to the data receiver
20b. The data receiver 20b subjects the first transmission signal to
amplification, frequency conversion, demodulation and
de-interleaving/error correcting decoding. As a result of such
processing, the data receiver 20b extracts a compressed echo signal from
the first transmission signal. The extracted compressed echo signal is
input to the echo expander 20c.

[0041] The echo expander 20c expands the compressed echo signal input from
the data receiver 20b by use of the compression parameter input from the
parameter determining unit 20g. As a result of such processing, the echo
expander 20c reproduces the digital RF echo signal. The reproduced RF
echo signal is input to the frequency down-conversion unit 20d. In the
case of the compressed sensing, the compression parameter input from the
parameter determining unit 20g includes a sample decimating rate or a
sample decimating rule, an RF pulse center frequency and an RF pulse
bandwidth. In other words, a parameter related to the RF echo signal is
used as the compression parameter. The echo expander 20c first inserts a
zero value into a thinned sample in accordance with the same random
decimating rule as that used for the sample decimating in the echo
compressor 15c, thereby restoring a sample vector having the same rate as
the RF echo signal. Further, amplitude/phase estimation is repeated using
frequency components in a frequency domain defined by the center
frequency and the bandwidth, thereby restoring sample vector indicating
the spectrum of the RF echo signal, that is, a digital RF echo signal.
The RF echo signal thus obtained is input to the frequency
down-conversion unit 20d. Alternatively, the echo expander 20c restores
the RF echo signal by the following principle. The sample vector output
by the ADC 15b of the probe unit 15 is represented by x=(x1, x2, . . . ,
xL) using an echo line number L (natural number). However,
xl=(1<=l<=L, an integral number) is xl={(x1,l), (x2,l), . . . ,
(xN,l)}T when the number of samples per echo line is N. Thus, x is an
N×L matrix.

[0042] A sample vector y after the decimating of samples in the echo
compressor 15c is represented by y=Φx using a sample decimating
matrix Φ. Φ is a matrix of M×N in which there only remain
rows corresponding to M samples left after decimating based on the sample
decimating rule out of a unit matrix of N×N. Alternatively, Φ
is a random number matrix of M×N. The RF echo signal is normally a
signal having a band that is significantly narrow for a sampling
frequency. Thus, the RF echo signal can be converted into a sparse signal
θ by fast Fourier transform (FFT) processing. θ=Fx when a
matrix expression corresponding to the FFT processing is represented by
F. In consequence, the sample vector y after the sample decimating is
represented by y=Φx=ΦF-θ. θ can be estimated by
solving the following optimization problem:

[0044] Here, the mathematical expressions in the case where the same
sample decimating rule is applied to all echo lines are shown. Otherwise,
a different sample decimating rule can be applied to each echo line. In
this case, a sample vector x is treated as x (x1; x2; . . . ; xL) (a
column vector having a length NL) in which x1s (1<=l<=L, an
integral number) are longitudinally joined together. Moreover, Φ is a
matrix in which Φ1s are lined on a principal diagonal and other
elements are zero. Φ1 is a matrix of M1×N in which there only
remain rows corresponding to M1 samples left after decimating out of the
unit matrix of N×N. Alternatively, Φ1 is a random number matrix
of M1×N.

[0045] The echo expander 20c finds a matrix x on the basis of the matrix
θ estimated as described above, and uses this matrix as the RF echo
signal.

[0046] The frequency down-conversion unit 20d multiplies the local signal
input from the variable frequency generator 20k by the RF echo signal
input from the echo expander 20c, and only passes a desired signal band
through filtering, thereby achieving the frequency down-conversion of the
RF echo signal. Magnetic resonance signal data thus obtained is input to
the image reconstructor 20e.

[0048] The pulse waveform generator 20p generates a base band pulse
waveform by use of a reference clock signal input from the fixed
frequency generator 20j. This base band pulse waveform is input to the
frequency up-conversion unit 20r.

[0049] The frequency up-conversion unit 20r multiplies the base band pulse
waveform input from the pulse waveform generator 20p by the local signal
input from the variable frequency generator 20k, and only passes a
desired signal band through filtering, thereby achieving the frequency
up-conversion of the base band pulse waveform. A signal thus obtained is
input to the radio frequency transmitter 19.

[0050] The sequence controller 20f determines the period of the RF pulse,
the kind of RF pulse, the center frequency of the RF pulse and the
bandwidth of the RF pulse in the imaging sequence, in accordance with
imaging conditions (e.g., a part to be imaged, the kind of probe used,
and a selected slice width) input by the operator by use of the operator
input unit 23. The center frequency of the RF pulse is reported to the
variable frequency generator 20k. The period of the RF pulse, the kind of
RF pulse and the bandwidth of the RF pulse are reported to the pulse
waveform generator 20p. Moreover, the part to be imaged, the kind of
probe used, the center frequency of the RF pulse and the bandwidth of the
RF pulse are reported to the parameter determining unit 20g. The center
frequency is directly input by the operator in accordance with the static
magnetic field that varies, for example, with time, or stored and
continuously used. The center frequency may be further adjusted by the
sequence controller 20f in accordance with the slice selected by the
operator.

[0051] The parameter determining unit 20g has, in association with a part
to be imaged and imaging conditions that are assumed, a table describing
a compression parameter suited to the part to be imaged and the imaging
conditions. In the case of the compressed sensing, tests are previously
conducted for a great number of imaging conditions different in the part
to be imaged, the kind of probe used, the center frequency of the RF
pulse and the bandwidth of the RF pulse that are input from the sequence
controller 20f. A sample decimating rate or sample decimating rule suited
to each of the great number of imaging conditions is determined to create
the above-mentioned table. The parameter determining unit 20g selects a
most suitable sample decimating rate or sample decimating rule in
accordance with the part to be imaged, the kind of probe used, the center
frequency of the RF pulse and the bandwidth of the RF pulse that are
input from the sequence controller 20f. The parameter determining unit
20g then inputs the sample decimating rate or sample decimating rule to
the echo expander 20c as a compression parameter together with the
bandwidth of the RF pulse and the center frequency of the RF pulse. The
parameter determining unit 20g also inputs the sample decimating rate or
sample decimating rule to the parameter transmitter 20h as a compression
parameter.

[0052] The parameter transmitter 20h subjects the compression parameter
input from the parameter determining unit 20g to proper error correcting
encoding/interleaving, modulation, frequency conversion, amplification
and filtering to generate a second transmission signal. This second
transmission signal is supplied to the transmission antenna 20i, and
thereby sent to the probe unit 15 via the second radio channel CH2.

[0053] The reference signal transmitter 20m subjects the reference clock
signal input from the fixed frequency generator 20j to modulation,
frequency conversion, amplification and filtering to generate a third
transmission signal. This third transmission signal is supplied to the
transmission antenna 20n, and thereby sent to the probe unit 15 via the
third radio channel CH3.

[0054] Thus, according to the MRI apparatus 100 in the first embodiment,
the ADC 15b digitizes the RF echo signal at a sampling rate twice the
maximum frequency of the RF echo signal or more. From a sample vector
thus obtained, a compressed echo signal obtained by decimating some of
the samples in the echo compressor 15c is sent by the data transmitter
15d. Therefore, even if the RF echo signal is not subjected to the
frequency down-conversion, the data transmitter 15d has only to be
adapted to a transmission data rate lower than the sampling rate which is
twice the maximum frequency of the RF echo signal or more. As a result,
the probe unit 15 does not have to be provided with any frequency
down-conversion unit, and a data rate requirement of the data transmitter
15d can be alleviated, thus allowing the reduction of the load on the
probe unit 15. In addition, the probe unit 15 needs the echo compressor
15c. However, the processing in the echo compressor 15c only includes
decimating some of the samples of the compressed echo signal in
accordance with the sample decimating rule. Therefore, the echo
compressor 15c can have a simpler configuration than that of the
frequency down-conversion unit.

[0055] Furthermore, according to the MRI apparatus 100 in the first
embodiment, compression can be achieved by use of a compression parameter
suited to the part to be imaged and the imaging condition, so that
optimum compression can be performed for various imaging targets/imaging
sequence.

[0056] The parameter determining unit 20g does not necessarily have to be
included in the control/imaging unit 20. The parameter determining unit
20g may be included in the probe unit 15. In this case, the
control/imaging unit 20 has only to be configured to report sequence
information to the probe unit 15 so that the parameter determining unit
20g may select a parameter accordingly.

[0057] The probe unit 15 may otherwise be configured to have a plurality
of RF probes 15a. In this case, as shown in FIG. 3, a plurality of ADCs
15b and a plurality of echo compressors 15c are provided to correspond to
the respective RF probes 15a. Moreover, a parallel/serial converter (P/S)
15j is provided. Compressed echo signals output from the echo compressors
15c are independently input to the parallel/serial converter 15j, and the
parallel/serial converter 15j rearranges these compressed echo signals
into a serial form. The parallel/serial converter 15j sends one obtained
compressed echo signal to the data transmitter 15d.

[0058] When the probe unit 15 has a plurality of RF probes 15a as shown in
FIG. 3, the image reconstructor 20e generates projection data in the
arrangement direction of the RF probes 15a on the basis of the magnetic
resonance signal data on the magnetic resonance signal received by
particular one of the plurality of RF probes 15a.

Second Embodiment

[0059] FIG. 4 is a block diagram of a detailed configuration of part of
the MRI apparatus 100 according to the second embodiment. In FIG. 4, a
characteristic configuration of the MRI apparatus 100 according to the
second embodiment is shown. Components which are not essential in the
second embodiment such as components associated with the control of the
gradient power supply 18 and the radio frequency transmitter 19 are not
shown. In FIG. 4, the same components as those in FIG. 2 are provided
with the same reference numbers and are not described in detail.

[0060] The probe unit 15 according to the second embodiment includes an RF
probe 15a, an analog-digital converter (ADC) 15b, an echo compressor 15c,
a data transmitter 15d, a transmission antenna 15e, a reception antenna
15f, a parameter receiver 15g, a reception antenna 15h and a reference
signal receiver 15i. That is, the probe unit 15 has the same
configuration in the first embodiment and the second embodiment.

[0061] In the meantime, the control/imaging unit 20 according to the
second embodiment includes a reception antenna 20a, a data receiver 20b,
an echo expander 20c, a frequency down-conversion unit (frequency D/C
unit) 20d, a sequence controller 20f, a parameter transmitter 20h, a
transmission antenna 20i, a fixed frequency generator (fixed f generator)
20j, a variable frequency generator (variable f generator) 20k, a
reference signal transmitter 20m, a transmission antenna 20n, a pulse
waveform generator 20p, a frequency up-conversion unit (frequency U/C
unit) 20r, an image reconstructor 20s and a parameter determining unit
20t. That is, the control/imaging unit 20 according to the second
embodiment has the image reconstructor 20s and the parameter determining
unit 20t instead of the image reconstructor 20e and the parameter
determining unit 20g in the control/imaging unit 20 according to the
first embodiment.

[0062] The image reconstructor 20s subjects magnetic resonance signal data
input from the frequency down-conversion unit 20d to image reconstruction
processing such as Fourier transform, and obtains image data (magnetic
resonance image data) for the magnetization of a desired nuclear species
in the subject 200. Alternatively, the image reconstructor 20s obtains
spectrum data for a desired nuclear spin, and thus obtains image data
indicating the desired nuclear spin. The image data thus obtained by the
image reconstruction is output to the display unit 22. The image
reconstructor 20s also calculates a signal-to-noise ratio (SNR) in the
reconstructed image data, and compares this SNR with a desired SNR in
order to instruct the parameter determining unit 20t to adjust a
compression parameter accordingly. In the case of the compressed sensing,
the SNR may be improved by decreasing the sample decimating rate. Thus,
the image reconstructor 20s inputs, to the parameter determining unit
20t, a command to decrease the sample decimating rate when the SNR is
lower than the desired SNR. On the other hand, the image reconstructor
20s inputs, to the parameter determining unit 20t, a command to increase
the sample decimating rate when the SNR is higher than the desired SNR.
The SNR is calculated and the increase or decrease of the sample
decimating rate is ordered, for example, after the acquisition of one
line echo in a calibration performed before a sequence, and before the
acquisition of a first line echo in a sequence. Moreover, the SNR is
calculated and the increase or decrease of the sample decimating rate is
ordered after the acquisition of an n-th line echo in the sequence, and
after the acquisition of an (n+1)-th line echo (note that n is 1 or more,
and is a natural number less than the number of lines in the sequence).
When image quality is improved by the synthesis of signals obtained by
performing a plurality of sequences, a calculated value of the SNR in an
image obtained in one sequence may be compared with the desired SNR set
as a value of the SNR in a desired image to order the increase or
decrease of the sample decimating rate used in a next sequence.

[0063] The parameter determining unit 20t has, in association with a part
to be imaged and imaging conditions that are assumed, a table describing
a compression parameter suited to the part to be imaged and the imaging
conditions. In the case of the compressed sensing, tests are previously
conducted for a great number of imaging conditions different in the part
to be imaged, the kind of probe used, the center frequency of the RF
pulse and the bandwidth of the RF pulse that are input from the sequence
controller 20f. A sample decimating rate or sample decimating rule suited
to each of the great number of imaging conditions is determined to create
the above-mentioned table. The parameter determining unit 20t selects a
most suitable sample decimating rate or sample decimating rule in
accordance with the part to be imaged, the kind of probe used, the center
frequency of the RF pulse and the bandwidth of the RF pulse that are
input from the sequence controller 20f. The parameter determining unit
20t further updates the selected sample decimating rate or sample
decimating rule to increase or decrease of the sample decimating rate in
accordance with a command input from the image reconstructor 20s. The
parameter determining unit 20t then inputs the updated sample decimating
rate or sample decimating rule to the echo expander 20c as a compression
parameter together with the bandwidth of the RF pulse and the center
frequency of the RF pulse. The parameter determining unit 20t also inputs
the sample decimating rate or sample decimating rule to the parameter
transmitter 20h as a compression parameter.

[0064] Thus, according to the MRI apparatus 100 in the second embodiment,
advantages provided by the MRI apparatus 100 in the first embodiment can
be also brought about. Moreover, according to the MRI apparatus 100 in
the second embodiment, the sample decimating rate can be adjusted to a
minimum value at which the SNR in the reconstructed image is equal to the
desired SNR.

[0065] Alternatively, the parameter determining unit 20t may only
determine a sample decimating rate or sample decimating rule in
accordance with a command input from the image reconstructor 20s without
taking into account the part to be imaged and the imaging conditions.

[0066] The parameter determining unit 20t does not necessarily have to be
included in the control/imaging unit 20. The parameter determining unit
20t may be included in the probe unit 15. In this case, the
control/imaging unit 20 has only to be configured to report, to the probe
unit 15, sequence information, and a command to increase or decrease the
sample decimating rate or an SNR so that the parameter determining unit
20t may select a parameter accordingly.

[0067] The probe unit 15 may otherwise be configured to have a plurality
of RF probes 15a. In this case, as shown in FIG. 5, a plurality of ADCs
15b and a plurality of echo compressors 15c are provided to correspond to
the respective RF probes 15a. Moreover, a parallel/serial converter 15j
is provided. Compressed echo signals output from the echo compressors 15c
are independently input to the parallel/serial converter 15j, and the
parallel/serial converter 15j rearranges these compressed echo signals
into a serial form. The parallel/serial converter 15j sends one obtained
compressed echo signal to the data transmitter 15d.

[0068] When the probe unit 15 has a plurality of RF probes 15a as shown in
FIG. 5, the image reconstructor 20s generates projection data in the
arrangement direction of the RF probes 15a on the basis of the magnetic
resonance signal data on the magnetic resonance signal received by
particular one of the plurality of RF probes 15a.

Third Embodiment

[0069] FIG. 6 is a block diagram of a detailed configuration of part of
the MRI apparatus 100 according to the third embodiment. In FIG. 6, a
characteristic configuration of the MRI apparatus 100 according to the
third embodiment is shown. Components which are not essential in the
third embodiment such as components associated with the control of the
gradient power supply 18 and the radio frequency transmitter 19 are not
shown. In FIG. 6, the same components as those in FIG. 2 are provided
with the same reference numbers and are not described in detail.

[0070] The probe unit 15 according to the third embodiment includes an RF
probe 15a, an analog-digital converter (ADC) 15b, an echo compressor 15c,
a data transmitter 15d, a transmission antenna 15e, a reception antenna
15f, a parameter receiver 15g, a reception antenna 15h and a reference
signal receiver 15i. That is, the probe unit 15 has the same
configuration in the first embodiment and the third embodiment.

[0071] In the meantime, the control/imaging unit 20 according to the third
embodiment includes a reception antenna 20a, a data receiver 20b, a
sequence controller 20f, a parameter determining unit 20g, a parameter
transmitter 20h, a transmission antenna 20i, a fixed frequency generator
20j, a variable frequency generator 20k, a reference signal transmitter
20m, a transmission antenna 20n, a pulse waveform generator 20p, a
frequency up-conversion unit (frequency U/C unit) 20r, an echo expander
20u and an image reconstructor 20v. That is, the control/imaging unit 20
according to the third embodiment does not have the frequency
down-conversion unit 20d in the first embodiment, and has the echo
expander 20u and the image reconstructor 20v instead of the echo expander
20c and the image reconstructor 20e.

[0072] The echo expander 20u expands data input from the data receiver 20b
by use of a parameter input from the parameter determining unit 20g to
obtain an RF echo signal. The echo expander 20u inputs this RF echo
signal to the image reconstructor 20v. In the case of the compressed
sensing, the data input to the echo expander 20u from the data receiver
20b is a thinned sample vector, and the parameter input from the
parameter determining unit 20g is a sample decimating rate or sample
decimating rule. When the sample decimating rate is provided, the same
random decimating rule as that used for the sample decimating in the echo
compressor 15c is generated. The echo expander 20c estimates an image
signal by the following principle in accordance with the sample
decimating rule, and inputs the estimated image signal to the image
reconstructor 20v.

[0073] The sample vector output by the ADC 15b of the probe unit 15 is
represented by x=(x1, x2, . . . , xL) using an echo line number L
(natural number). However, xl=(1<=l<=L, an integral number) is
xl={(x1,l), (x2,l), . . . , (xN,l)}T when the number of samples per echo
line is N. Thus, x is an N×L matrix.

[0074] A sample vector y after the decimating of samples in the echo
compressor 15c is represented by y=Φx using a sample decimating
matrix Φ. Φ is a matrix of M×N in which there only remain
rows corresponding to M samples left after decimating based on the sample
decimating rule out of a unit matrix of N×N. Alternatively, Φ
is a random number matrix of M×N.

[0075] In conventional processing that images the sample vector x output
by the ADC 15b, linear processing such as frequency conversion,
filtering, decimation and FFT processing is performed to obtain a matrix
expression m of an image. However, the size of the matrix m is P×P
wherein P is the number of pixels in one side of the image. m=Fx when a
matrix expression corresponding to the above-mentioned linear processing
is represented by F. θ=Ψm=ΨFx if a conversion matrix for
performing a basis conversion of the image matrix into a sparse matrix
θ is represented by T.

[0076] Consequently, the sample vector y after the sample decimating is
represented by y=Φx=Φ(ΨF)-θ. θ can be estimated
by solving the following optimization problem:

[0078] Here, the mathematical expressions in the case where the same
sample decimating rule is applied to all echo lines are shown. However, a
different sample decimating rule can be applied to each echo line. In
this case, a sample vector x is treated as x (x1; x2; . . . ; xL) (a
column vector having a length NL) in which xls (1<=l<=L, an
integral number) are longitudinally joined together. Moreover, Φ is a
matrix in which Φ1s are lined on a principal diagonal and other
elements are zero. Φ1 is a matrix of M1×N in which there only
remain rows corresponding to M1 samples left after decimating out of the
unit matrix of N×N. Alternatively, Φ1 is a random number matrix
of M1×N.

[0079] When the compressed sensing is applied in an image domain, the
matrix m can be obtained from the estimated matrix θ by use of a
relation θ=Ψm. Thus, part of processing for conversion from an
echo signal to an image signal which is part of the processing in the
image reconstructor 20v is included.

[0080] The image reconstructor 20v finds the matrix m on the basis of the
matrix θ estimated in the echo expander 20u, and finds image data
as data that indicates this matrix.

[0081] Thus, according to the MRI apparatus 100 in the third embodiment,
advantages provided by the MRI apparatus 100 in the first embodiment can
be also brought about. Moreover, the third embodiment enables a smaller
calculation amount because the size of the matrix to be estimated is
generally smaller than that in the first embodiment.

[0082] The probe unit 15 may otherwise be configured to have a plurality
of RF probes 15a. In this case, as shown in FIG. 7, a plurality of ADCs
15b and a plurality of echo compressors 15c are provided to correspond to
the respective RF probes 15a. Moreover, a parallel/serial converter 15j
is provided. Compressed echo signals output from the echo compressors 15c
are independently input to the parallel/serial converter 15j, and the
parallel/serial converter 15j rearranges these compressed echo signals
into a serial form. The parallel/serial converter 15j sends one obtained
compressed echo signal to the data transmitter 15d.

Fourth Embodiment

[0083] FIG. 8 is a block diagram of a detailed configuration of part of
the MRI apparatus 100 according to the fourth embodiment. In FIG. 8, a
characteristic configuration of the MRI apparatus 100 according to the
fourth embodiment is shown. Components which are not essential in the
fourth embodiment such as components associated with the control of the
gradient power supply 18 and the radio frequency transmitter 19 are not
shown. In FIG. 8, the same components as those in FIGS. 2, 4, 6 are
provided with the same reference numbers and are not described in detail.

[0084] The probe unit 15 according to the fourth embodiment includes an RF
probe 15a, an analog-digital converter (ADC) 15b, an echo compressor 15c,
a data transmitter 15d, a transmission antenna 15e, a reception antenna
15f, a parameter receiver 15g, a reception antenna 15h and a reference
signal receiver 15i. That is, the probe unit 15 has the same
configuration in the first embodiment and the fourth embodiment.

[0085] In the meantime, the control/imaging unit 20 according to the
fourth embodiment includes a reception antenna 20a, a data receiver 20b,
a sequence controller 20f, a parameter transmitter 20h, a transmission
antenna 20i, a fixed frequency generator 20j, a variable frequency
generator 20k, a reference signal transmitter 20m, a transmission antenna
20n, a pulse waveform generator 20p, a frequency up-conversion unit
(frequency U/C unit) 20r, a parameter determining unit 20t, an echo
expander 20u and an image reconstructor 20w. That is, the control/imaging
unit 20 according to the fourth embodiment does not have the frequency
down-conversion unit 20d in the control/imaging unit 20 according to the
first embodiment, and has the echo expander 20u, the image reconstructor
20w and the parameter determining unit 20t instead of the echo expander
20c, the image reconstructor 20e and the parameter determining unit 20g.

[0086] The echo expander 20u and the parameter determining unit 20t have
the functions described in the third embodiment and the second
embodiment.

[0087] The image reconstructor 20w has the function of obtaining image
data similarly to the image reconstructor 20v according to the third
embodiment, and the function of inputting a command to increase or
decrease the sample decimating rate to the parameter determining unit 20t
similarly to the image reconstructor 20s according to the second
embodiment.

[0088] Thus, according to the MRI apparatus 100 in the fourth embodiment,
advantages provided by the MRI apparatuses 100 in the first to third
embodiments can be also brought about.

[0089] The probe unit 15 may otherwise be configured to have a plurality
of RF probes 15a. In this case, as shown in FIG. 9, a plurality of ADCs
15b and a plurality of echo compressors 15c are provided to correspond to
the respective RF probes 15a. Moreover, a parallel/serial converter 15j
is provided. Compressed echo signals output from the echo compressors 15c
are independently input to the parallel/serial converter 15j, and the
parallel/serial converter 15j rearranges these compressed echo signals
into a serial form. The parallel/serial converter 15j sends one obtained
compressed echo signal to the data transmitter 15d.

[0090] The reference clock signal and the compression parameter may be
transmitted via one radio channel. That is, for example, a signal
obtained by modulating the reference clock signal using a code vector
that indicates the compression parameter is sent from the control/imaging
unit 20 to the probe unit 15. For example, on-off-keying (OOK) or
amplitude-shift-keying (ASK) can be used for this modulation. Thus, the
probe unit 15 extracts the code vector that indicates the compression
parameter from the above-mentioned signal sent from the control/imaging
unit 20, and reproduces the reference clock signal.

[0091] While certain embodiments have been described, these embodiments
have been presented by way of example only, and are not intended to limit
the scope of the inventions. Indeed, the novel embodiments described
herein may be embodied in a variety of other forms; furthermore, various
omissions, substitutions and changes in the form of the embodiments
described herein may be made without departing from the spirit of the
inventions. The accompanying claims and their equivalents are intended to
cover such forms or modifications as would fall within the scope and
spirit of the inventions.